The present invention relates to video keyers used for video special effects, and more particularly to a keyer with variable shaping for combining a fill video signal with a background video signal by combining the benefits of luminance and linear keying.
In video special effects systems key signals are generated to effectively "cut holes" in one video signal to insert another video signal. Two traditional key signal generators are a luminance keyer and a linear keyer. The luminance keyer usually has two controls--a clip and a gain control. The gain control determines the amount of gain used in the key processing, typically in a range of 5-150, and the clip control determines a reference level used in the key processing. The linear keyer has the same controls as the luminance keyer except that the amount of gain used in the key processing is much lower and includes unity gain, typically in a range of 0.2-20 so that the high and low levels enclose the unity gain function. Additionally luminance keyers require special processing called "shaping." The shaping of luminance keys is important as a method of reducing visual artifacts caused by discontinuities in a key control output signal between an intermediate ramp and high and low levels when clipped at the output. For linear keyers the shaping is undesirable since the input key signal is to be faithfully reproduced, and the input key signal may already be shaped. These two keyers are separate functions with a separate set of controls for each keyer.
Therefore what is desired is a single keyer that produces both a luminance key signal and a linear key signal from the same set of controls so that the transition between the two key signal functions is continuous and relatively undetectable to an operator.